The invention is based on a fuel injection device for internal combustion engines, such as a fuel injector defined hereinafter.
With such generic fuel injection devices and in particular unit fuel injectors, a greater degree of freedom is attained, in terms of intervening to govern and control the entire course of events involved in injection, than is possible with conventional injection devices using a distributor or in-line type of injection pump. This greater freedom is partly due to operating at a higher injection pressure, which some high-speed direct-injection engines require. Moreover, the engine camshaft is typically used to drive the pump plunger, so that existing drive mechanisms are put to additional use, which reduces both expenses and power losses.
In the unit fuel injectors of this generic type, as well as in unit fuel injectors not having an earlier priority, a great number of variants using a suitable intermediate or secondary plunger are known. In these pumps, the supply onset, which determines the injection onset, and the supply quantity intended for injection are defined via the control of the fuel quantities in the pump chamber and in the pressure chamber. The cam used to drive the unit fuel injector is usually available on the engine camshaft. The fuel quantities are controlled as a function of angle and time; angle control refers to control of the plunger stroke by the drive cam, while timing control refers to the opening of a fuel line by the control valve. The cam drive path can be divided into three segments: a slowly dropping intake stroke segment (run-off edge), a repose segment (cam base circle) and an ensuing steep compression stroke segment (compression stroke edge) that changes into the intake stroke segment. This compression stroke segment of the cam path is relatively short and steep, to attain the desired injection effect, which requires fast pump plunger motion. The intake stroke segment, contrarily, is relatively flat to enable the suction effect at the required control times, and in combination with the repose segment is also relatively long, to make as much control time available as possible, since in this plunger operating position, usually only pure timing control is available, via the control valve.
With these generic injection devices, the control of the supply quantity and adjustment of the supply onset also takes place without a governor rod or centrifugal adjuster of the kind typical for in-line pumps, but instead with the aid of the control valve, which at the correct time meters a suitable quantity of fuel into one of the chambers, jointly with the action of the control edges of the pump plunger or intermediate piston and with check valves. As used here, the term check valves includes valves of any conceivable type that prevent a backward flow, including valves lacking a valve seat and instead embodied as slide valves.
Because in the generic injection device the intermediate plunger arrives at a fixed end position defined by a stop at the end of the compression stroke, the same outset position is attained for this intermediate plunger prior to each intake stroke, so that this position corresponds to a definable cam path point. Angle control during governing, via the cam path and plunger control edges, is thus more easily controlled by being coupled with the time control determined by the control valve.
Control problems in such systems are in fact much greater than can be described here, especially because of spurious oscillations that are difficult to control. Spurious oscillations arise in the mechanical pump plunger drive, but they have a constant amplitude and constant frequency, and are distinguished solely by their phase location with respect to the injection. Although such spurious oscillations are virtually uncontrollable, a quantitative estimate of their effect on fuel quantity deviation can be made. In simplified terms, it can be assumed that in control by the control edges, the control quantity remains uninfluenced by the spurious oscillation. The situation is different with magnet valve control, where the critical spurious frequency is very high. At a constant amplitude, it can result in a fuel quantity error of several per cent, and this error is uncontrollable. The error can be divided in half by controlling the fuel quantity with a combination of control edges and the magnet valve, namely if one end is controlled with the control edge and the other with the magnet valve. On the other hand, triggering of a magnet valve by an electronic control unit should not be omitted. These comments apply not only to a magnet valve, but to a valve controlled in any way whatever.
In a known unit fuel injector (U.S. Pat. No. 4,235,374), the supply quantity and supply onset are determined by the opening and closing of a magnet valve disposed in the metering line to the pump chamber; a filling line that is unaffected by the magnet valve leads to the compression chamber of the unit fuel injector, and a check valve is disposed in this line. During a first intake stroke segment of the pump plunger, the magnet valve remains closed, so that with the pump chamber now closed, the intermediate plunger is pulled along with the pump plunger, causing fuel at low pressure to flow via the filling line and the check valve into the pressure chamber. Once a supply quantity intended for the injection has been metered, the magnet valve in the supply line opens, so that fuel at low pressure now flows via the supply line into the pump chamber, whereupon the intermediate plunger, because of the hydraulic forces then engaging it, remains where it had been.
In this known unit injector, the magnet valve then remains upen until the onset of the ensuing compression stroke, so that during the remaining intake stroke fuel flows into the pump chamber and fills it. Even during the repose segment of the cam path, the magnet valve remains open. The supply quantity flowing into the pressure chamber is thus determined by the opening instant of the magnet valve, by means of which the prestorage of the supply quantity in the pressure chamber is terminated. This manner of supply quantity determination is relatively imprecise, because it is indirect, so that besides the control errors noted above, many other kinds of factors can affect it, such as leakage in the line and valves, changes in flow conditions such as the opening forces of the check valve, and the varying temperature of the fuel and variations in the feed pump pressure.
Another disadvantage of this kind of indirect control is that especially at higher rpm, the supply line and filling line are affected both by the inertia of the intermediate plunger and by throttling action. For instance, the throttling effect of the magnet valve with respect to the pump chamber and of the check valve with respect to the compression chamber must be matched very precisely, as must the operative surface areas of the intermediate plunger in combination with the low fuel pressure and the spring urging the intermediate plunger toward the pressure chamber in this known unit injector. Since the dynamic force of the sluggish intermediate plunger varies as a function of rpm, and to determine the injection quantity, the intermediate plunger must be stopped during its intake stroke, the resultant function equations are extraordinarily complex and include an rpm-dependent factor, making theoretical predetermination of such a system impossible.
There are also considerable disadvantages in controlling the supply onset in this known unit fuel injector, because the magnet valve is not closed until a certain compression stroke segment of the pump plunger has been completed, and injection into the engine combustion chamber cannot take place until after that, or in other words not until the available fuel volume trapped in the pump chamber moves the intermediate plunger, to feed the fuel quantity pre-stored in the pressure chamber to the injection nozzle. In this first compression stroke segment, the corresponding portion of the fuel located in the pump chamber is pumped back to the fuel source, via the magnet valve and the supply line. In dynamic conditions, which vary with the rpm and are particularly influential at high rpm, this fuel flow reversal, or in other words this displacement back and forth of a volume of fuel, can lead to highly variable acceleration forces, which can cause an error in the desired instant of supply onset. This disadvantage is virtually unremediable, because the desired instant of supply onset is only conditionally dependent on the rpm, so that errors notoriously due to the rpm cannot be eliminated by using the rpm as the parameter. This problem is further exacerbated because in some cases the instant of supply onset must be varied as a function of other engine parameters, such as load, but in each case still as a function of the rpm.
Yet another disadvantage of this known unit injector is that the magnet valve must be embodied as a high-pressure valve, to be able to withstand the pressures arising during the compression stroke. Moreover, it must close during the compression stroke; although during return pumping there is as yet no injection pressure, still the pressure that prevails then is far higher than the feed pump pressure. This return pumping pressure varies with rpm within certain limits, which again has an effect on the closing process, with a tendency of dragging the supply onset in the "late" direction. In every case, such a magnet valve must also be designed for the injection pressure, that is, a maximum possible fuel pressure, which not only entails high production costs but also means high consumption of electricity during use. Since extraordinarily short control times are necessary, pre-controlled magnet valves are virtually out of the question. Since the pressure difference between the intake side and the high-pressure side is approximately 1:100, relatively slight deviations in timing or cross section have the effect of causing large errors in fuel quantity.
Another disadvantage of this known system is that because the supply quantity is controlled purely as a function of rotational angle, that is, indirectly via the magnet valve, any change in throttling action in the filling line to the pressure chamber results in an error in the quantity control. This is especially critical because the supply quantities in idling and at full load vary by a ratio of 1:15, and each must be metered if possible between the minimum and maximum rpm. To enable sufficiently accurate metering of a full-load quantity at maximum rpm, a minimum instake stroke segment is thus needed. Taking these conditions into account, it has already been proposed (German Patent Application No. 37 00 352.6), in a fuel injection device and in particular a unit fuel injector of the type described at the outset above, that the supply line controlled by the control valve be controlled by the pump plunger, and for this supply line to be disconnected from the pump chamber and connected to the filling line of the pressure chamber during a first intake stroke segment of the pump plunger, and then during a later segment of the intake stroke and during the repose segment of the cam driving the pump plunger to re-connect this supply line to the pump chamber, after first disconnecting the supply line from the filling line. This improves the control substantially. Since the entire quantity control, namely the control of both the supply quantity and the quantity determining the supply onset, is shifted to the run-off edge of the cam, the area available there for control can be enlarged, by correspondingly reducing the size of the compression stroke edge. The compression stroke edge now serves purely for high-pressure injection, and can therefore advantageously be embodied as very steep, which has advantages especially for the course of injection. Since neither the supply quantity nor the supply onset is controlled on the high-pressure side, the course of injection can advantageously be varied arbitrarily, without having to vary the supply quantity as well. By reducing the effective cam angle for the high-pressure edge, the angle for the run-off edge or the base circle of the cam can be correspondingly increased, so that more time is available for the intake stroke and repose segments, and the run-off edge can also have a correspondingly flatter course. In this way, the run-off edge can be made about 5 to 7 times flatter than the compression stroke edge. The metering time becomes correspondingly longer, and the critical spurious frequencies become correspondingly lower. It is possible for the spurious frequency to move into the vicinity of the injection frequency, which predominantly determines the phase location and amplitude of a neighboring spurious frequency. Because of the longer available control time, deviations in the fuel quantity to be controlled that are due to the magnet valve are also lessened, because such direct errors decrease in accordance with the increased time that is available. Entirely aside from that, a substantially less expensive low-pressure valve can be used as the magnet valve, with which still higher switching frequencies are attainable without problems. Moreover, because of the longer time that is available, such a magnet valve can also be used for direct control of both the supply quantity and the quantity determining the supply onset.
In this known proposal, reversing the fuel supply line from the pressure chamber to the pump chamber and back again to the pressure chamber is effected via the pump plunger itself, which thus acts as a mechanical control slide. Naturally an equivalent provision is also conceivable, in which a control slide driven synchronously with the pump plunger takes on this task.
In the above known proposal, the control valve is closed upon the onset of the intake stroke and opens to initiate the metering of the supply quantity into the compression chamber, whereupon the intermediate plunger already executes the required stroke because of the suction exerted by the pump plunger. The greater the intended supply quantity, the earlier the magnet valve will open. In each case, the pumping into the pressure chamber is interrupted whenever the pump plunger blocks the supply line. The magnet valve continues to be open, however, as long as the pump plunger reverses the fuel flow; thus in the repose segment, when the pump plunger has established the connection of the supply line to the pump chamber, fuel flows into the hollow chamber that has meanwhile formed in the pump chamber until the magnet valve closes again. This volume that has flowed in is definitive for the supply onset; that is, the earlier the magnet valve closes, the later the injection supply onset is effected.
Although the intermediate plunger always begins its intake stroke at the end stop, and the magnet valve advantageously need open and close only once per pump cycle, the otherwise floating intermediate plunger results in limit situations, in the connections among the chambers and conduits involved, that make it difficult to control the sources of error, in particular those caused by spurious oscillations but also those due to the magnet valve.